Corundum is a crystalline form of aluminium oxide (Al
3) typically containing traces of iron, titanium, vanadium and chromium. It is a rock-forming mineral. It is a naturally transparent material, but can have different colors when impurities are present. Transparent specimens are used as gems, called "ruby" if red and "padparadscha" if pink-orange. All other colors are called "sapphire", e.g., "green sapphire" for a green specimen.
The name "corundum" is derived from the Tamil word Kurundam which originates from the Sanskrit word Kuruvinda meaning ruby.
Because of corundum's hardness (pure corundum is defined to have 9.0 on the Mohs scale), it can scratch almost every other mineral. It is commonly used as an abrasive on everything from sandpaper to large machines used in machining metals, plastics, and wood. Some emery is a mix of corundum and other substances, and the mix is less abrasive, with an average Mohs hardness of 8.0.
In addition to its hardness, corundum is unusual for its density of 4.02 g/cm3, which is very high for a transparent mineral composed of the low-atomic mass elements aluminium and oxygen....LESS